1. Field of the Invention
This invention relates to the growth of single crystals of lead magnesium niobate-lead titanate (PMN-PT) grown near the morphotropic phase boundary. These ferro-relaxor single crystals are useful in various military, medical, and industrial sonic transduction applications.
2. Brief Description of the Prior Art
Piezoelectric materials are materials that expand and/or contract when an electric field is applied to them. Conversely, they also will produce an electric field across themselves if a mechanical force is applied to them. Common uses for materials possessing the piezoelectric effect are in gas lighters, high frequency speakers, weighing devices, and micro-positioners, to mention a few. The piezoelectric effect occurs in materials with an asymmetric crystal structure. When an external force is applied, the charge centers of the crystal structure separate creating electric charges on the surface of the crystal. As mentioned above, the effect is also reversible. Electric charges on the crystal will cause a mechanical deformation of the crystal. Quartz, turmalin, and seignette are common natural piezoelectric materials.
One area of emphasis in the art of piezoelectric materials has been directed to making polycrystalline ceramic piezoelectrics because physical properties can be tailored to a given application. Common ceramic piezoelectric materials are lead-zirconate-titanate (PZT) and lead-magnesium-niobate (PMN). Piezoelectric materials typically deform linearly in response to an applied electric field. Because the strains in piezoelectrics are small, piezoelectric actuators are used mainly in speakers or precision micro-positioning applications where small and precise motion is needed.
Piezoelectric ceramics are currently the material of choice for ultrasonic transducer applications, offering relatively high coupling, a wide range of dielectric constants, and low dielectric loss. These merits translate into transducer performance in the form of relatively high sensitivity, broad bandwidth, impedance matching, and minimal thermal heating.
Relaxor-ferroelectrics are similar to piezoelectrics except that strain is produced by a second order electrostrictive effect as opposed to the first order effect. The most widely studied relaxor material is the PMN-PT solid solution system. This material is among the most widely used so called, “smart” materials. Achieving high quality single crystals of solid solution lead magnesium niobate-lead titanate (PMN-PT) greatly enhances the electromechanical properties of the PMN-PT as compared to the ceramic form. Obtaining high quality single crystals of PMN-PT is a significant objective in the art of piezoelectric materials.
PMN-PT has been grown by several techniques including flux, top seeded flux, and vertical Bridgman methods. To date, the vertical Bridgman method has been the most successful in producing large single crystals of PMN-PT. As shown in FIG. 1, a crucible system 10 with a flat bottom section 11 may used in the vertical Bridgman method. In this example, a piece of an oriented crystal seed 16 is placed at the bottom 11 of a platinum crucible 12 with a hermetically sealed lid 18 to seed crystal growth. During crystal growth, a crystal 14 forms from solidification of the melt, and the orientation of the seed crystal is propagated throughout the entire crystal. An important feature of the vertical Bridgman method is that the crystal shape is controlled by the crucible shape. However, the Bridgman method has several disadvantages, including crucible-melt compatibility (chemical and thermal), the presence of mechanical stress (crucible/crystal contraction), compositional segregation (stoichiometry, impurities), crucible cost, and removal of the crystal from the crucible.
Another approach is to grow single crystal of PMN-PT is shown in FIG. 2, which shows a crucible system 20 with a conical bottom section 21. In this case, the use of a seed is optional. As the melt solidifies at the bottom 21 of the inverted cone shape of a platinum crucible 22, several grains of various orientations form simultaneously. One of the grains will grow faster than the others and eventually choke off the growth of the other grains as the solidified crystal propagates upward in the inverted conical section of the crucible. The result is the formation of a crystal having a major grain 24 and one or more minor grains 26. This process is known as grain selection.
The grain selection method has several disadvantages. The seed is usually made of the same material of the crystal being grown and, therefore, has the same melting point. Special precautions and crucible design must be employed to prevent the melting of the seed during the initial period of the growth. With a conical crucible bottom 21, a large portion of the total length of the grown crystal is taken up by the conical section rather than the desired full diameter cylindrical section. In addition, a portion of the conical section consists of several grains before the point of a single crystal is obtained.
In current practice, crucibles are made from very thin platinum metal. After the starting materials are placed in the crucible, a platinum lid is welded to the crucible to form a hermetically sealed container. The sealed crucible prevents lead oxide vapor from escaping from the crucible during growth. The wall of the crucible is thin because the coefficient of thermal expansion of the platinum is greater than that of the PMN-PT. As the crystal cools, the contraction of the crucible is greater than that of the crystal resulting in the creation of compressive forces within the crystal. The thin crucible wall partially relieves these stresses.
Such methods for producing PMN-PT single crystals are disclosed in U.S. Pat. No. 5,998,910 to Park et al., the disclosure of which is herein incorporated by reference.
There remains a need for a method to produce PMN-PT single crystals that does not rely on seeds or a conical bottomed crucible, but achieves a single crystal of specific orientation from a flat-bottomed crucible in such a way that it is easily removed from the crucible.
These and other advantages of the present invention will be clarified in the description of the preferred embodiment taken together with the attached drawings in which like reference numerals represent like elements throughout.